1. Field of the Invention
The invention relates to a projection objective for projecting a pattern arranged in the object plane of the projection objective into the image plane of the projective objective with the aid of ultraviolet light of a prescribed operating wavelength.
2. Description of the Related Art
Photolithographic projection objectives have been in use for several decades for producing semiconductor components and other finely structured components. They serve the purpose of projecting patterns of photomasks or reticles, which are also denoted below as masks or reticles, onto an object, coated with a photosensitive layer, with a very high resolution on a reducing scale.
In order to generate ever finer structures of the order of magnitude of 100 nm or below, an attempt is being made to enlarge the image-side numerical aperture (NA) of the projection objectives beyond the values currently attainable into the range of NA=0.8 or above. Moreover, use is being made of ever shorter operating wavelengths of ultraviolet light, preferably wavelengths of less than 260 nm, for example 248 nm, 193 nm, 157 nm or below. At the same time, an attempt is being made to fulfil the increasing demands on the projectability with the aid of purely refractive, dioptric systems which are advantageous by comparison with catadioptric systems with regard to design and production. In the context of wavelengths which are becoming ever shorter, however, only a few sufficiently transparent materials, whose Abbe constants are relatively close to one another, are still available. This raises problems for a partial achromatization, and even more so with a complete achromatization of the projection objectives, that is to say the far-reaching avoidance or reduction of chromatic aberrations. In particular, it is difficult to provide very high aperture systems with adequate small chromatic aberrations. Furthermore, with rising aperture and an additional need for improved imaging qualities together with unchanged object and image fields there is an increase in the dimension of the projection objective in all three spatial directions. In particular, the increase in volume of the optical lens material increases the cost of such projection objectives disproportionately in relation to the gain in reducing structural width.